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Dense-packed cellulose in exterior walls

Corgan1 | Posted in Energy Efficiency and Durability on

I have a 100+ year old home and there is currently no insulation in the walls. There is vinyl siding on the exterior and I think it was installed over the old wood siding. I am considering having dense-packed cellulose blown into my 2×4 exterior walls. Am I going to have a moisture problem if I do so? I’ve read conflicting opinions. I would appreciate any advice you can offer.

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  1. GBA Editor
    Martin Holladay | | #1

    No, your walls will not have any new moisture problems if you insulate them with cellulose.

    At a home with painted clapboard siding, adding cellulose insulation to previously empty stud bays sometimes contributes to premature paint failure. But your house has vinyl siding -- perhaps the best type of siding when it comes to exterior drying -- so you shouldn't have any problems.

    Here is a link to an article with more information: How to Install Cellulose Insulation.

  2. bunney | | #2

    T.S., that's going to be quite the upgrade going from no insulation to insulation! Thorough description of techniques and issues in the link Martin included.

    I too had a moisture related question about dense-packed cellulose, Martin, that you or others may want to weigh in on. I am building in zone 6, 46 degrees latitude in a cold-winter, humid summer climate in North Central Minnesota. Budget limits led to an under-insulated wall assembly:

    LP Smart Board Siding
    House wrap (crinkled to facilitate drainage)
    1" rigid exterior insulation
    OSB sheathig
    2x6 wall cavity with studs 16 inches on center
    blown in fiberglass or dense packed cellulose
    smart membrane vapor retarder
    5/8 sheetrock

    Thus, at certain times of the winter I will have cold sheathing. We have discussed potential for moisture accumulation in our assembly which will have to dry to the inside. Builder's standard practice is to blow in fiberglass for insulation, in part because the fiberglass will not hold moisture,.

    All things being equal, I prefer cellulose for increased air control and sound dampening.

    Q.1 Am I better to go with blown in fiberglass or dense packed cellulose (with both having similar R value) for to reduce risk of moisture problems?
    Q.2. Is vapor barrier needed? Will it increase risk for moisture problems in wall cavity?

  3. GBA Editor
    Martin Holladay | | #3

    A "smart" vapor retarder like MemBrain will lower the risk of this wall assembly -- an assembly which breaks the rules stipulating minimum R-values for exterior rigid foam.

    Another way to limit the risk is to keep your indoor relative humidity low during the winter.

  4. bunney | | #4

    It makes me sick to have boxed myself into a budget corner where I indeed broke the rules for my wall assembly.

    1. That said, am I correct in understanding that risk is more with that the framing members absorbing moisture during portions of the winter than it is with wall cavity insulation?

    If so, does it make any difference moisture wise whether I use blown in fiberglass or dense packed cellulose?

  5. GBA Editor
    Martin Holladay | | #5

    I don't think that there is a clear answer to your question, although I don't doubt that some GBA readers may be willing to suggest one.

    As I wrote in my recent blog ("Hygric Buffering and Hygric Redistribution"), "There is no easy way to compare the advantages and disadvantages of blown-in fiberglass insulation with the advantages and disadvantages of cellulose insulation. Water is more likely to drain quickly through fiberglass than cellulose, and that’s good — right? And damp fiberglass dries more quickly than cellulose — also good, right?

    "But on the other hand, cellulose provides both a hygric buffer and hygric redistribution — characteristics that are absent from fiberglass insulation — so the cellulose must be preferable, right?

    "Assessing the effects of these pluses and minuses is difficult even for building scientists."

  6. Expert Member
    Dana Dorsett | | #6

    The risk to the sheathing is much higher than the risk to the framing members. Being more thermally conductive than the insulation and extending toward the warmer interior, the average temp of the framing members is higher than that of the sheathing, which means they don't take on as much moisture. By the time you have moisture damage to the framing the sheathing would have long-since turned to mush after years of extreme moisture cycling.

    MemBrain is cheap insurance, and should do the trick. For more money you could go with Intello Plus, which is more vapor tight and more physically rugged to boot. Drying rates aren't as fast as MemBrain, but it's still over 10 perms when subjected to very high humidity, and quite a bit more vapor tight when the proximate air is dry.

    Cellulose would be more protective of the sheathing & framing than fiberglass, since it's hollow fiber structure can buffer more than an order of magnitude more moisture without damage, dripping liquid water, or losing it's insulating function. It wicks moisture from the sheathing, and stores it. But cellulose hangs onto that moisture until later in the season, and it's buffering capacity is not a substitute for controlling the vapor permeance to the interior. You don't want it to store so much moisture that it can't dry sufficiently before the temperatures reach mold-growth levels in spring. With fiberglass the peak moisture content of the sheathing is likely to be higher, but it'll also dry faster when outdoor temperatures warm up. As long as the wood dries before the temps support mold growth you're golden, but you'll be starting from a higher moisture content.

  7. bunney | | #7

    You are good teachers, Dana, and Martin. Thanks.. Best, Randy

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